The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, regulations for an eco-friendly vehicle such as a hybrid electronic vehicle (hereinafter, referred to as HEV) including a plug-in hybrid electronic vehicle (PHEV) include motor regulations for a driving motor generating driving power together with an engine.
The motor regulations include net power regulations regulating maximum output of a motor.
Accordingly, maximum output of the hybrid starter and generator (hereinafter, referred to as HSG) applied to the HEV and rotated by the engine separately from the driving motor is also limited due to an influence of the motor regulations. Here, the HSG charges a high voltage battery at the time of engine start and during driving, and controls power generation amount according to a load amount of the engine or the motor.
Hereinafter, MTPA is an abbreviation of “maximum torque per ampere” that means maximum torque per unit current, represented by an MPTA curve, and applied to field weakening or flux weakening. The field weakening is a method of increasing torque by armature current that is increased by decreasing counter electromotive force by weakening field magnetic flux in an counter electromotive force increase phenomenon due to increase in speed and a torque decrease phenomenon due to decrease in armature current after an allowable terminal voltage of the motor is reached.
Specifically, the HSG control implemented in the HEV is a method of implementing maximum output limitation of the HSG as torque and power limitation by limiting an MTPA region to constant torque, and limiting regions after the MTPA region to constant output. At this point, the maximum output limitation is performed within up to 2% as compared to HSG specification due to the influence of the net power regulations.
The method of simultaneously limiting the torque and the output of the HSG as described above is called a torque-output limiting method, and is performed in a manner that a motor control unit controlling the driving motor is connected to a hybrid control unit. A practical logic of the torque-output limiting method is divided into an HSG driving mode and an HSG regeneration mode.
For example, the HSG driving mode of the torque-output limiting method is a method in which torque limitation is performed up to about 2,100 rpm after the engine starts, and thereafter, the method is shifted to the output limitation. Meanwhile, the HSG regeneration mode of the torque-output limiting method is a method in which a zero torque control state is maintained after the engine starts, and engine energy is applied as regenerative torque of the HSG in an RPM decrease state due to engine off following engine idle, and the HSG regeneration mode is performed for about five seconds.
In the torque-output limiting method, engine characteristics may not be maximally reflected when implementing the HSG regeneration mode.
For example, the engine RPM is high at the time of engine idle and changed to be low right before the engine is turned off, and becomes zero when the engine is turned off. However, in the torque-output limiting method, regenerative torque output of the HSG after the low RPM is controlled in a zero torque control state after the engine starts. As a result, the HSG gradually increases initial regenerative torque output in a limited state, then the increased torque output is maintained constantly and decreased, and the battery is charged using the engine energy except for the limited initial regenerative torque output.
We have discovered that, in view of the engine characteristic of repeating operation and stop, fuel efficiency may be additionally improved by decreasing friction loss of the engine energy, but in the current torque-output limiting method, since the initial regenerative torque output is limited, the fuel efficiency may not be additionally improved.